Split channel direct-current insertion network



March 8, 1949. A. v. BEDFORD SPLIT CHANNEL DIRECT-CURRENT INSERTIONNETWORK Filed March 17, 1945 Patentecl Mar. 8, 1949 SPLIT CHANNELDIRECT-CURRENT INSERTION NETWORK Alda V. Bedford, Princeton, N. J.,assignor to Radio Corporation of America, a corporation of DelawareApplication March 17, 1945, Serial No. 583,343

(Cl. P18-7.3)

3 Claims. l

This invention relates generally to signal correction networks and moreparticularly to an improved method of and means for correcting forspurious low frequency modulation components in a signal communicationsystem and for substantially eliminating spurious high frequency signalcomponents normally introduced by D.C. insertion networks.

The invention will be described by reference to circuits suitable foruse in a television receiving system wherein D.-C. insertion is requiredfor the received video signals to compensate for spurious low frequencymodulation and low frequency infidelity of the received signal toprovide an accurate value of the mean brightness of the receivedpicture.

It is well known that the D.C. and low frequency components of atelevision picture which are lost or distorted in transmission may bereconstructed at the receiver by means of a capacitor seriallyinterposed between the video ampliler and the television kinescope tubein cpmbination with a diode peak detector connected in shunt with thekinescope circuit. In this combination the diode draws current duringthe peaks of the synchronizing signals, thereby creating a control biasfor the control electrode of the kinescope which provides the desiredD.C. signal component. However, noise peaks also cause the diode to drawcurrent, thereby providing false values of D.C. insertion. Also, sincethe vertical synchronizing pulses of the television signal are ofsubstantially longer time duration than the horizontal synchronizingpulses, the control bias values for the different types of synchronizingsignals vary substantially. The longer duration vertical synchronizingsignal tends to depress abruptly the control signal level. Hence, inconventional circuits the vertical synchronizing pulses even may notproperly actuate the vertical synchronizing circuits.

The instant invention contemplates the use of a split-channel D.C.restoring network wherein the low frequency or D.C. spurious componentsof the received signal are effectively restored, and at the same timethe spurious high frequency components normally introduced by the D.C.restoring circuit are substantially eliminated from the picture signalemployed to modulate the intensity of kinescope tube cathode ray. Thisfeature is accomplished by combining, through complementary filtersystems, only the low-frequency components of the restored picturesignal with complementary high-frequency components of the initialsignal to form therefrom a complete, substantially accurate, videosignal which may be employed to modulate the kinescope tube and toactuate the synchronizing circuits. The complementary filtering is suchthat the sum of the signal response from the two filter systems isuniform for all frequencies in a selected fre-` quency band. Thus, inoperation the vertical synchronizing pulses tend to maintain the samesignal level as the horizontal synchronizing pulses, since thevariations in the levels of the synchronizing pulses in conventionalsystems comprise spurious components which are predominately highfrequency in character.

The instant invention is an improvement upon the system described inapplicants copending U. S. application, Ser. No. 583,342, led March 17,1945, and now abandoned, wherein a low-pass filter is seriallyinterposed between the series capacitor and the shunt-connected diodepeak detector of a conventional D.C. insertion network, in order thatthe D.C. insertion signal may be substantially independent of the noisesignals.

Three embodiments of the present invention are described and illustratedherein. The first embodiment comprises a conventional D.C. insertionnetwork and a single resistive-capacitive lter network for providingcomplementary filtering of the received and the restored signalcomponents. A second embodiment of the invention utilizes separatehigh-pass and low-pass complementary resistive-capacitive filternetworks for the received and the restored signal componentsrespectively, and a coupling network for combining the complementarilyfiltered signal components.

The third embodiment of the instant invention combines the novelfeatures of applicants ccpending application, identified heretofore,with the novel features of the first embodiment of the instant inventionin a circuit for providing restored video signals for both modulationandsynchronization of the deflection of a kinescope tube in response totelevision signals.

Among the objects of the invention is to provide an improved method ofand means for correcting for spurious low-frequency modulation ofcommunication signals. Another object is to provide an improved methodof and means for reconstructing the low-frequency components in acommunication signal. Another object is to provide an improved method ofand means for D.C. insertion in a signal communication system. A furtherobject of the invention is to provide an improved method of and meansfor reducing the effect of noise signals in a D.C. insertion circuit fora signal communication system. An additional object of the invention isto provide an improved D.C. insertion split-channel network for a signalcommunication system, wherein the D.C. insertion circuit and thereceived signal circuit are coupled through complementary filter systemsto provide a substantially restored communication signal. A furtherobject of the invention is to provide an improved split-channel D.C.insertion network wherein the correcting 4 signal components and thereceived signal components are combined through separate complementaryhigh-pass and low-pass filter systems to provide a restoredcommunication signal; Another object of the invention is to provide animproved D.C. insertion network for a signal communication system,wherein the restoring signal components and the received signalcomponents are combined through a plurality of complementary filtersystems to provide restored communication signals for modulating anddeflecting the cathode ray beam of a television kinescope tube.

The invention will be described in greater detail by reference to theaccompanying drawing of which Figure 1 is a schematic circuit diagram ofa first embodiment thereof, Figure 2 is a graph of a typical televisionsignal having spurious low-frequency signal components or improperloW-frequency response, Figure 3 is a graph of said television signalwherein the spurious low-frequency components have been substantiallyeliminated, but additional spurious high frequency components have beenintroduced by a conventional D. C. restoring network, Figure 4 is aschematic circuit diagram of a second embodiment of the invention, andFigure 5 isa schematic circuit diagram of a third embodiment of theinvention. Similar reference characters are applied to similar elementsthroughout the drawing.

Referring to Figure 1 of the drawing, signals from a television receiveror other signal source are applied to input terminals 3, 5. A smallvcapacitor 1 and a diode peak detector 9 are connected in series betweenthe input terminals 3, 5. A high value shunt resistor II is connectedacross the diode 9. A lter network comprising a serially-connected smallfilter capacitor I3 and filter resistor I5, are connected across thei'irst capacitor 1. Output terminals I'I, I9 are connected to thejunction of the filter capacitor I3 and filter resistor I5 and toground, which corresponds to the input terminal 5.

In operation, received signals a, as shown in the graph of Figure 2, areapplied, through a high-pass filter comprising the series lter capacitorI3 and the effectively shunt-connected filter resistor I5, to the outputterminals I'I, I9 whereby the low-frequency components of the receivedsignal are deleted. Also, due to the normal action of the D. C.inserting network comprising the rst capacitor 1, the diode 9 and the.shunt resistor II, low-frequency restoring wave of Figure 3.

signal components are provided at the point b, which correspond to thesignals shown in the graph of Figure 3. These signals are asubstantially true replica of the original signal except that theaverage maximum signal level is depressed at the region 1J correspondingto the occurrence of the vertical synchronizing pulses, as explainedheretofore. The signal b thence is applied to the output terminals andcombined with the signal a by passing it through the series lterresistor I5 and the effectively shunt-connected filter capacitor I3,Thus, substantially only complementary high-frequency components of thereceived signal a. may be combined at the output terminals to provide asubstantially restored communication signal c.

The partially restored signal b is produced as follows: Only thehigh-frequency and mediumfrequency components of the received signal aare transmitted through the small first capacitor 'I to the-diode peakdetector 9 since the load provided by the shunt resistor II greatlyattenuates the low-frequency components of the signal a. However, thediode peak detector 9 draws current on each peak of the synchronizingsignals when the anode of the diode becomes positive with respect toground. This current generates a bias voltage wherein the received waveis depressed below the zero axis in a manner wherein the peak values ofthe signal exceed zero by a relatively small amount which is determinedby the value of the shunt-resistor II'. This action tends to provideuniform peak signal level as shown by the envelope 29 of the signalAbsolute uniformity of the signal pulse levels would indicate idealrestoration of the low-frequency components of the transmitted signal.

However, as indicated at the portion o of the graph of Figure 3, thevertical synchronizing pulses being many times longer than thehorizontal synchronizing pulses, the diode conducts for a much greaterpercentage of the time during the interval of the vertical synchronizingpulse. Therefore, additional bias is generated by the diode 9, whichtends to depress the signal peak values below the maximum level of thehorizontal synchronizing pulses. This condition implies that the wave isgiven a spurious component which although repeated sixty times persecond is comprised principally of much higher harmonic frequencycharacteristics.

Since the spurious signal component caused by the vertical synchronizingpulses is essentially of relatively high frequency character, this highfrequency spurious component may be effectively eliminated by means ofthe `low-pass lter interposed between the diode and the output circuit.Also, since the spurious low-frequency components of the received waveare substantially eliminated by the high-pass filter interposed betweenthe input and output circuits, the combination of the output signalsderived from the low-pass and high-pass filter action tends to reproducethe originally transmitted signal since the filters are complementary incharacter.

The action of the filter comprising the lter capacitor I3 and nlterresistor I5 in providing complementary response to the signals appliedto the outer terminals thereof, may be explained as follows: Byconsidering the point b as temporarily grounded, it is evident thatsignals reaching the point c from the point a will have their lowfrequencies greatly attenuated. Similarly, by considering the point atemporarily grounded, signals transmitted from b to c will have theirhigh frequency components greatly attenuated. By the inherent nature ofthe circuit, the filtering action for the two signals is actuallycomplementary. Hence, a substantially perfect correction of the receivedvideo signal will be provided.

The circuit of Figure 4 is similar to the circuit of Figure 1 exceptthat separate high-pass and low-pass filter networks 2 I, '23 aresubstituted for the common filter network I3, I5 of the circuit ofFigure 1. The output of the high-pass filter 2| is coupled to the outputterminal I1 through a rst isolating resistor 25. Similarly, the outputof the low-pass network 23 is coupled to the output terminal I1 througha second isolating resistor 21. The frequency characteristics of thehigh-Dass and low-pass filters 2l and 23, respectively, are madecomplementary by proper selection of the filter parameters. Theoperation of the circuit of Figure 4 is substantially identical to theoperation of Figure l described heretofore.

As explained heretofore, the third embodiment of the inventionillustrated in Figure 5 combines the novel features of the instantinvention with those of applicants copending application identifiedheretofore- Video signals derived from one of the tubes 3I of aconventional television radio receiver are applied, through a low-passfilter network 33 for attenuating noise components, to a voltage divider35. Signals applied to the voltage divider 35 also are coupled through asmall capacitor 1 and a second low-pass filter network 31 to the diodedetector 9 across which is shunted a second high resistance voltagedivider I I'. The output circuit of the tube 3| also is coupled througha first series filter capacitor I3 to the control electrode 4I of aconventional kinescope television receiving tube 43. The cathode of thediode peak detector 9 is coupled through a first series filter resistorI5 to said control electrode 4I of the kinescope tube 43. Hence,received video signals are applied to the control electrode 4I of thekinescope tube 43 by passing them through the high-pass filter I3, I 5.

Also the received signals derived from the ouput of the first low-passfilter 33 are coupled through the capacitor 1 and the second low-passfilter 31 to the cathode of the diode peak detector 9, in the samemanner as described in said copending application, whereby the D.C.insertion circuit including the diode 9 is substantially unaffected bynoise signal components. The lowfrequency components of the D.C.insertion signal thence are coupled to the control electrode 4I of thekinescope tube 43 through the low-pass filter comprising the filterresistor I5 and capacitor I3, as explained heretofore with respect tothe circuit of Figure 1.

Thus, the control electrode 4I of the kinescope tube 43 receivescomplementary high frequency and low frequency signal components fromthe received signal circuit and the D.C. insertion network,respectively.

The cathode and first anode of the kinescope -tube 43 are supplied withoperating potentials from a third voltage divider 45 which is connectedacross a source of operating potential, not shown.

The desired high-frequency components of the received signal are appliedto the control electrode of a first limiter amplifier tube 41 through asmall coupling capacitor I3' which is connected to an adjustable contacton the rst voltage divider 35. Similarly the low-frequency compo- Rilnents of the D.C insertion signal are derived from an adjustable contacton the second voltage divider II' and applied to the control electrodeof the first limiter amplifier 41 through a second lter resistor I5'.Thus, the second filter capacitor I 3' and second filter resistor I 5comprise a second complementary filter system for applying restoredsignals to the control electrode of the first limiter-amplifier 41 inthe same manner as described heretofore with respect to the signalsapplied to the -control electrode 4I of the kinescope tube 43, exceptthat said signals are filtered by the rst low-pass filter 33.

The limited signals derived from the first limiter-amplifier 41 arefurther limited and amplifled in a second limiter-amplifier tube 49. Thelatter tube actuates conventional kinescope deecting circuits 5I whichare connected to the defiecting elements of the kinescope tube 43 toaccomplish the required synchronization of the cathode ray deflection inthe kinescope tube. The tap on the second voltage divider 35 usuallyshould be adjusted to a somewhat lower level than the tap on the thirdvoltage divider II to provide optimum waveform for the signals which areapplied to the first limiter-amplifier tube 41. Simultaneous adjustmentof the taps on both of the voltage dividers provides selection of theprop er synchronizing signal level. Since the cathodes of the triodelimiters are coupled together, the grid-cathode bias on the rst tube 41determines one limiting level, and the grid-cathode bias on the secondtube 49 determines the other limiting level for the synchronizing signalpulses, in a manner well known in the art. Thus, the circuit providesD.C. restoring action for both the modulating and deflecting signalsactuating the kinescope tube.

The various embodiments of the invention disclosed herein comprise anovel split-channel D.C. restoring network for television or othercomplex signal systems wherein complementary filter systems are employedfor combining complementarily signal components derived from the signalreceiving circuits and the D.C. insertion circuits for providing asubstantially perfect replica. of the transmitted signal.

I claim as my invention:

1. A D.C. inserting network for connection between a communicationsignal source and utilization means, said network including a firstcapacitor serially interposed between said signal source and saidutilization means, a second capacitor, a low-pass filter and aunilaterally conducting circuit serially connected with said secondcapacitor, said serially connected second capacitor, low-pass filter andunilaterally conducting circuit being connected in shunt with saidsignal source, a resistor connected between said unilaterally conductingcircuit and said utilization means, said unilaterally conducting circuitproviding a bias voltage for said utilization means responsive to peaksignal magnitude, and said first capacitor and said resistor comprisingelements of a complementary filter system for substantially eliminatingspurious high frequency signal components introduced by said biasvoltage circuit, a second utilization means, and second complementaryfilter means connecting said signal source and said unilaterallyconducting circuit to said second utilization means.

2. The network as set forth in claim 1 including a low-pass filterinterposed between said second capacitor and said signal source, saidsecond complementary filter means comprising a resistor :Maasmo- 7 8andmacondensensaid resistor'beingserially" coni A y n nectedv from saidunilaterally conducting. circuit REFERENCES CITED to secon-d utilizationmeans; and said last named Thel following references are of recordin thecondenser being serially'connected froznpsatidsig". le 0f this Patentinalsourcethrough said` last named 10W-pass lter 5 UNITED. STATES PATENTStosad' second signal utilization-source.

3. 'I'he network of claim 2V inf whichfsaid. last Number Name Date namedresistor is connected throughapotentiom. 2,208,374 Lew-is July 16, 1940eter to said unilaterally conducting cincuit, `and 2,24Ch281v Ba11ard.'Apr, 29, 1941 saidiast named condenser is connected fromY said m2526fb1'72lv BatchelOr N0V-25, 1941 signal source through a. second.potentiometer;

ALDA V. BEDFORD;

